Constraining shallow S-wave velocity structure beneath the Azores–Madeira–Canaries region from Rayleigh-wave ellipticity analysis using UPFLOW data

Seafloor sediment layers strongly influence seismic signals recorded by ocean-bottom seismometers through reverberations, velocity reduction, and waveform amplification. These effects can significantly bias seismic observations, limiting investigations of the oceanic crust and mantle. While direct drilling and active-source seismic surveys provide robust constraints on sediment structure, they are not always feasible in areas instrumented solely with passive seafloor seismometers. In this study, we estimate Rayleigh-wave ellipticity from both ambient seismic noise and earthquake recordings using a polarization-based H/V approach that isolates elliptically polarized Rayleigh waves. Rayleigh-wave ellipticity derived from OBS data shows clear correlations with water depth and sediment thickness. The combined ellipticity curves are inverted using the Neighbourhood Algorithm to constrain crustal shear-wave velocity structure beneath the OBS stations. Our inversion results indicate sedimentary cover thicker than ~2 km beneath the Madeira region, closer to the continent, whereas relatively thin sediment layers are observed near the Azores region. The resulting crustal thickness and shear-wave velocity models across the Azores–Madeira–Canaries region provide a useful reference for future seismic investigations in this region, including studies based on the UPFLOW data.